Melphalan uptake by ovarian carcinoma cells isolated from patients with clinically documented disease was reduced 7-fold by incubation of cells with radiolabeled drug in the ascitic fluid when compared to incubation of cells in a balanced salt solution containing albumin and glucose. Automated amino acid analysis of ascitic fluid indicated that it contained physiological concentrations of a number of amino acids, notably leucine (100-150 micromolar) and L-glutamine (500-600 micromolar), which reduce melphalan uptake by ovarian carcinoma cells to levels obtained with ascitic fluid. Cytotoxicity evaluations were in agreement with transport data.